The present invention generally relates to liquid separation components, systems and methods. More particularly the present invention relates to liquid flotation separation components, systems and methods that employ one or more gasses for separating particulate matter and other contaminants from carrier liquid streams.
It is often necessary to remove contaminants from liquid. For example, the need to remove particles, colloids, solvent and oil from wastewater is desirable in many settings.
Typically, such contaminants are water borne. These streams are typically treated using coagulants and flocculants to form sludge, which is separated from the liquid.
Dissolved air flotation (DAF) systems are often used to separate particulate material from liquids such as wastewater. The systems typically employ the principle that bubbles rising through a liquid attach to and carry away particles suspended in the liquid. As bubbles reach the liquid surface, the attached particles coalesce to form a froth that is collected.
Traditional DAF systems typically introduce small air bubbles into the lower portion of a relatively large tank filled with the liquid to be treated. The air bubbles rise through the liquid and attach to particles in it. The tank includes an outlet through which treated liquid passes at a flow rate consistent with the inlet rate of the liquid plus a fraction for air entrainment.
DAF system processing times and contaminant removal efficiencies typically depend on the residence time of the bubbles in the solution. The residence time, in turn, is affected by bubble size, bubble buoyancy, the depth of the bubbles within the liquid, and the amount of turbulence in the liquid. As footprint increases, the probability increases that particles will contact the bubbles during the residence time available within the tank. In addition, relatively large footprints allow the bubbles sufficient time to rise through the depth to reach the free liquid surface. As a result, traditional DAF systems employ relatively large and costly tanks having correspondingly large “footprints”.
The very size of such systems increases the period of time between control adjustment and effect. This is because water going by the adjustment point, for example a polymer inlet upstream of the DAF, requires over half an hour, and usually over an hour, to reach the outlet of the DAF. Thus, there is a substantial delay (i.e. ½ to 1 hour response time) before the effect of the adjustment can be ascertained so as to inform the next adjustment. Thus, these systems lack real-time or even near real-time control. In the event the processing produces a treated effluent stream that is outside operating requirements, the long response time results in production of many gallons of out-of-specification wastewater.
This is especially true under circumstances in which the DAF unit receives flows from several dissimilar processes. This is a common occurrence. Many times the separate flows make up varying fractions of the total flow entering the DAF unit. Floor drains from a canning floor, for example, may carry a fairly small quantity of drained liquid most of the time and large flows during wash downs. Although the normal flow may be similar to the flow from the boiler operation, during wash downs it will exceed the boiler flow. Thus, the character of the composite flow that reaches the DAF can commonly change from one minute to the next. Unless adjustments are made to the DAF process, usually via adjustment of chemical dosages, the contaminant removal efficiency will vary and may degrade below requirements. A need exists for the ability to make real time or near real time adjustments that respond to shifts in the character of the streams to be treated. The large tank size of the typical DAF tank is in part due to the need to flatten these stream variations.
In an effort to reduce the tank size for a DAF system, one proposal disclosed in U.S. Pat. No. 4,022,696 employs a rotating carriage and floc scoop. The carriage directs an inlet solution substantially horizontally along a flow path to increase the path length for bubble travel, and correspondingly increasing the residence time. However, the rotating carriage and scoop create turbulence that slows bubble rise. Unfortunately, while the tank size reduction is set forth as an advantage, the problem with performance tied to residence time still remains.
Another proposal, disclosed in U.S. Pat. No. 5,538,631, seeks to address the turbulence problem by incorporating a plurality of spaced apart and vertically arrayed baffles. The baffles include respective vanes angularly disposed to re-direct the flow of liquid from an inlet positioned at the bottom of the tank. Liquid flowing through the tank deflects upwardly as it traverses the vanes, purportedly reducing the extensity and intensity of turbulence generated near the inlet to the tank.
While this proposal purports to reduce the turbulence problem relating to bubble residence time, the redirected fluid still appears to affect bubbles rising in other areas of the tank, and influences the residence time of such bubbles. Moreover, the proposal fails to address the basic problem of DAF performance being dependent on the need to accomplish bubble-to-particle-adhesion during bubble rise. This increases the residence time needed to complete separation.
In an effort to overcome the limitations in conventional DAF systems, air-sparged hydrocyclones (ASH) have been proposed as a substitute for DAF systems. One form of air-sparged hydrocyclone is disclosed by Miller in U.S. Pat. No. 4,279,743. The device typically utilizes a combination of centrifugal force and air sparging to remove particles from a fluid stream. The stream is fed under pressure into a cylindrical chamber having an inlet configured to direct the fluid stream into a generally spiral path along a porous wall. The angular momentum of the fluid generates a radially directed centrifugal force related to the fluid velocity and indirectly with the radius of the circular path. The porous wall is contained within a gas plenum having gas pressurized to permeate the porous wall and overcome the opposing centrifugal force acting on the fluid.
In operation, the unit receives and discharges the rapidly circulating solution while the air permeates through the porous wall. Air passing through the walls of the porous tube is sheared into the fluid stream by the rapidly moving fluid flow. Micro-bubbles formed from the shearing action combine with the particles or gases in the solution and float them toward the center of the cylinder as a froth in a vortex. The centrally located froth vortex is then captured and exited through a vortex finder disposed at the upper end of the cylinder while the remaining solution exits the bottom of the cylinder.
In operation, however, a substantial portion of the froth tends to become re-entrained on the liquid leaving the hydrocyclone instead of exiting the top. In addition, froth exiting the top usually has a substantial fraction of water that must then be subjected to lengthy dewatering for decanting back into the process upstream of the hydrocyclone.
One variation in the general ASH construction, as described in U.S. Pat. Nos. 4,838,434 and 4,997,549, includes employing a froth pedestal at the bottom of the cylinder to assist directing the froth vortex through the vortex finder. Another ASH modification includes replacing the vortex finder and froth pedestal with a fixed splitter disposed at the bottom of the cylinder and having a cylindrical knife edge. The edge is positioned to split the helically flowing solution into components dependent upon the specific gravity of the components. As above, the ASH systems tend to suffer from relatively large amounts of solution typically remaining in the froth, and significant particle concentrations often remaining in the solution.
Morse, et al, disclosed in U.S. Pat. No. 6,106,711 a system using a hydrocyclone that differs from the above by the absence of a froth pedestal and vortex finder and by the fact that both the froth and the liquid exit the hydrocylone together. In addition, the system relies on a downstream tank with vanes that are slanted from the vertical so as to separate the bubble-particle aggregates from the mass of the liquid stream. Morse, et al, also disclosed in U.S. Pat. No. 6,171,488 a system using a hydrocyclone that differs from U.S. Pat. No. 6,106,711 in that the hydrocyclone makes a submerged entry into the downstream tank.
Although for both of these patents the assembly is small compared to DAF systems, and so provides for near-real-time control, the assembly is a single unit that requires a sizeable location and is large enough to require special equipment to move. It also cannot accommodate the sequential introduction of more than one additive that must be thoroughly mixed with the stream before the introduction of the next additive. For example, it is desirable to adjust pH before adding polymeric flocculants so that high doses of the latter are avoided. In addition, a higher number of extremely fine bubbles would improve flotation. For these Morse inventions, there are not many variables that can be adjusted to optimize performance, so the manufacture of the systems often must be customized to the waste stream to be treated.
In addition, there can be problems scaling up to flows over 100 gallons per minute. At such flows, the momentum of the water is such that bubbles form that are over ½ inch in diameter. These bubbles interfere with flotation by being too large to aggregate with flocs and by creating cavitation, noise and vibration in the piping. In addition, the bubble size distribution begins at 20+ microns and does not bond to the small particles.
Therefore, the prior art has not solved the essential problems of large footprints, process control, flexibility, and small (nanometer) bubble size. Thus, a continuing need exists for a flotation separation system with components that need not be near one another so that space constraints can be accommodated. The need also exists for a method of simply and economically creating large quantities of the optimal size bubble needed at each step of the flocculation and flotation process. The need also exists to be able to easily vary the types and order of additives to minimize doses and interference with downstream additives. An additional need exists for a separation system that reduces the amount of additives needed per unit volume of liquid to be treated, which would reduce ongoing operational costs. The flotation separation system and method of the present invention satisfies these needs and provides other related advantages.